Java- abstract modifier / Chapter -3/ Access Modifier

Java- abstract modifier

the abstract is a modifier applicable for classes and methods but not for variables.

1 . abstract method:-
If we don't know about implementation even though we have to declare the method then we should go for abstract method.
In the abstract method, the only declaration is available but not the implementation that's why abstract method declaration should end with "; " semicolon.


example:-
public abstract void m1( ) ;           correct
public abstract void m1( ){ }          incorrect

Child class is responsible to provide implementation for parent class abstract method.

example:-

abstract class Parent
{
    public  abstract int salary() ;
}


class Child1 extends Parent
{
    public int salary( )
    {
           return 70000;   
    }
}
class Child2 extends Parent
{
    public int salary( )
    {
           return 60000;   
    }
}

By declaring abstract method in the parent class we can provide guidelines to the child class such that which methods compulsory child class has to implement.

 

abstract method never talks about implementation.

If any modifier talks about implementation then it will be an illegal combination with the abstract modifier.

The following are illegal combinations of modifiers for the method with respect to "abstract".

 

2. abstract class:-
For any java class if we are not allowed to create an object then we have to declare that class as "abstract class".
If we know only partial implementation then we should go for abstract class.
Hence for abstract classes, instantiation is not possible.

example:-

abstract class Test
{
     public static void main(String [] args)
     {
         Test  t =new Test( );
      }
}
Compile-Time Error
Test is abstract, cannot be instantiated.

1.If a class contain at least one abstract method then compulsory we should declare that class as "abstract" otherwise we will get compile time error.

Reason:-

 If a class contain at least one abstract method then the implementation is not complete, hence it is not recommended to create objects.

To restrict object instantiation compulsory we should declare that class as "abstract".

 

 

2.Even though the class does not contain any abstract method still we can declare that class as "abstract".

if we don't want object instantiation.

So that abstract class can contain 0 number of the abstract method also.

example:-

 

HttpServlet class is abstract but it does not contain any abstract method. 

 

 

Important Syntax Errors 

example 1

 

class Test

{

       public void m1( );

}

 

Compile Time Error: missing method body, or declare as abstract.

 

 

example 2

 

class Test

{

       public abstract void m1( ){}

}

 

Compile Time Error: abstract methods cannot have a body.

 

 

example 3

 

class Test

{

       public abstract void m1( );

}

Compile Time Error: Test is not abstract and does not override abstract method m1( ) in Test.
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